Christopher is 3 times as old as Omar. Sixteen years ago, Christopher was 7 times as old as Omar. How old is Omar now?
Solution: We can use the given information to write down two equations that describe the ages of Christopher and Omar. Let Christopher's current age be $c$ and Omar's current age be $o$ The information in the first sentence can be expressed in the following equation: $c = 3o$ Sixteen years ago, Christopher was $c - 16$ years old, and Omar was $o - 16$ years old. The information in the second sentence can be expressed in the following equation: $c - 16 = 7(o - 16)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $o$ , it might be easiest to use our first equation for $c$ and substitute it into our second equation. Our first equation is: $c = 3o$ . Substituting this into our second equation, we get: $3o$ $-$ $16 = 7(o - 16)$ which combines the information about $o$ from both of our original equations. Simplifying the right side of this equation, we get: $3 o - 16 = 7 o - 112$ Solving for $o$ , we get: $4 o = 96.$ $o = 24$.